Orthogonal frequency division multiplexing is widely used technique of transmission in the RF domain where it allows mitigating signal fading in multi-path propagation. The present invention discloses the use of orthogonal frequency division multiplexing in optical links and, in particular, in fiber communications.
In optical OFDM systems each WDM channel the optical carrier is directly modulated by a complex RF signal that can be construed as a linear combination of M separate digitally modulated RF signals at frequencies fm such that fm=m/T h power where T is the period of modulation. Thus the total symbol rate of the transmitted information is M/T. In the text we shall refer to the frequencies fm as “subcarriers”.
In modern optical communication systems, a coherent detection technique is implemented, which provides improved sensitivity compared with traditional direct detection schemes. Typically coherent detection is used with phase-shift-keying (PSK) data transmission. The present invention is also focused on M-PSK, and in the preferred embodiment, QPSK (quadrature PSK) data transmission. However this does not limit the scope of the invention, and various types of data modulation can benefit from the disclosed invention.
In a coherent receiver, the QPSK incoming optical signal is mixed with a strong local oscillators to produce in-phase (I) and in-quadrature (Q) outputs. I and Q components of the output optical signal are converted into electrical signals by a set of photodetectors. In the preferred configuration four balanced photodetectors are used to recover QPSK encoded data.
Data transmission multiplexing light of two orthogonal polarizations via the same optical channel allows doubling the data rate. At the receiver side, the orthogonal polarizations are split by a polarization beam splitter, and the light of each orthogonal polarization is detected separately.
U.S. patent application Ser. No. 10/405,236 by Roberts et al. discloses a nonlinearity compensation system applicable to WDM optical transmission. It considers many WDM channels and essentially performs numerically operations of complex amplitudes of the signals in all channels. However it is completely impractical to assure perfect control of the relative optical phase shifts between different WDM channels as they travel through their respective fibers (shown as 10a in FIG. 2 of '236) and through the MUX. The latency of the system is quite long, it includes travel time through the link, plus processing, which is typically a few milliseconds. Over that time the relative phases of different channels significantly shift. Such system requires the adjustment of their parameters at a rate of GHz. Alternatively such system may be used with a look-up table (LUT). The calculations show that the size of such LUT and the power consumptions make this solution impractical. Furthermore, since the whole link is dispersive in the system described in '236, the disclosed compensation does not provide sufficient link performance.
High capacity optical signal transmission is affected by the channel nonlinearity and dispersion, which leads to the limitations in the channel capacity, transmission distance and error rates. The present invention addresses this problem of the signal distortion caused by nonlinear effects.